In a conventional Package-on-Package (PoP) process, a top package, in which a first device die is bonded, is bonded to a bottom package through solder balls. The bottom package may also include a second device die bonded therein. The second device die may be on the same side of the bottom package as the solder balls.
Before the bonding of the top package to the bottom package, a molding compound is applied on the bottom package, with the molding compound covering the second device die and the solder balls. Since the solder balls are buried in the molding compound, a laser ablation or drilling is performed to form holes in the molding compound, so that the solder balls are exposed. The top package and the bottom package may then be bonded through the solder balls in the bottom package.
In the bonding process, the solder balls in the top package need to be accurately aligned to the solder balls in the bottom package, so that the solder balls in the top package and the solder balls in the bottom package may be reflowed to join to each other. To perform the alignment, alignment marks are formed in the reflow boat, wherein bottom packages are placed in the openings in the reflow boat. The solder balls in the bottom packages are located through locating the alignment marks in the reflow boat. Alternatively, the solder balls in the top package and the solder balls in the bottom package are used as alignment marks.
The conventional alignment schemes suffer from drawbacks. For example, the process variation in the sawing of the bottom packages may cause the variation in the alignment of the solder balls. The surface of the solder balls may be oxidized, making the identification of the solder balls difficult.